This invention is primarily concerned with a method for treating aqueous cyanide-containing liquors with an ozone-containing gas to substantially destroy the cyanide content therein. The present invention is also concerned with destroying substantial amounts of cyanates in such liquors formed by the initial oxidation of cyanides.
Simple cyanides as well as complex cyanide compounds are present in the waste effluents from certain chemical operations such as metal plating processes, photographic processes, mining operations, and metal refining processes. Because of the toxic nature of these cyanides and the potential hazard of contaminating water supplies, these effluents cannot be discharged into rivers or lakes without pretreatment to reduce cyanide levels. This contamination problem is not avoided by discharging such effluents on land since the cyanides can eventually pass through the soil and reach water supplies. In order to protect the environment and the public at large, federal and state governments have prescribed limits on the levels of cyanides in effluents discharged from such chemical operations.
Cyanates, which are formed by the initial oxidation of cyanides, are less toxic than cyanide but nonetheless present a potential danger to the environment. Government regulations have not yet been promulgated for cyanate discharge but it is expected that such regulations will be forthcoming. Therefore, there is a substantial need for an efficient method of destroying cyanides in wastewater effluents to acceptable levels and also for a method which reduces the cyanate content in the effluent finally discharged to the environment.
In the past, attempts at substantially destroying cyanides contained in wastewater effluent streams have either been too inefficient or too expensive for large scale commercial treatments.
For example, toxic cyanides can be precipitated as insoluble heavy metal compounds by the addition of a heavy metal salt to a cyanide-containing liquor. Unfortunately, complete elimination of cyanides is not accomplished by this method and it requires costly and bulky equipment.
Oxidation of cyanides to cyanates and then to carbon dioxide and nitrogen by alkaline chlorination has also been described in the art. One of the initial products, cyanyl chloride, however is also toxic and destruction of it is slow which often results in incomplete treatment of the effluent. Moreover, the addition of agents to control pH adds to the total dissolved solids in the effluent.
The use of peroxides to oxidize cyanide has also been suggested but is considered commercially prohibitive due to the high cost of materials.
Another problem shared by the above methods is their inability to break down metal-complexed cyanides such as iron cyanide.
More recently, ozone-containing gases such as ozone-air, ozone-oxygen and ozone-air and oxygen, either alone or in combination with a U.V. light treatment, have been employed as cyanide treatment agents for wastewater effluents because they are relatively inexpensive to produce and are efficient oxidants for cyanides. Ozone rapidly oxidizes cyanide to cyanate if a high mass transfer of ozone to the effluent is accomplished. The oxidation of cyanate to gaseous nitrogen and carbon dioxide however, is kinetically controlled and therefore requires significant additional contact time with ozone. Metal complexed cyanides such as iron cyanides are not usually destroyed by ozonation.
In U.S. Pat. No. 3,920,547 to R. L. Garrison et al., a method for the destruction of cyanides, particularly cyanides complexed with iron in an aqueous cyanide solution is provided comprising contacting the solution with an ozone-containing gas while simultaneously irradiating the aqueous cyanide solution with ultra-violet light. The method is preferably carried out while maintaining the pH of the solution between 5 and 9 at temperatures of between 30.degree. C. and 70.degree. C. It is also preferred to contact the aqueous cyanide solution and ozone-containing gas in a plurality of separate zones, one atop the other in a tower, countercurrently or by parallel flow, with the simultaneous irradiation with UV light being carried out in at least one of the separate contact zones, preferably in the last zone where the cyanide ion concentration is a minimum and reaction rate must be enhanced.
To provide for a more efficient dispersion of ozone-containing gas, each zone in the tower can be equipped with a means to provide small bubbles in the solution such as a mixer, porous stone diffuser, ozone ejector or other suitable means to obtain a satisfactory mass transfer of ozone from the gas to the liquid phase.
In U.S. Pat. No. 3,732,163 to W. Lapidot, a process and apparatus for treating industrial waste streams is described employing a plurality of ozone treatment zones wherein a major portion (70% to 95%) of the liquid to be treated is introduced into the upper portion of a first ozonation zone and the remaining portion of the liquid is introduced into a second ozonation zone, each zone comprising a packed tower. An ozone-containing gas mixture enters the bottom of the first zone, contacts the liquid therein and is discharged to the lower portion of the second zone. The treated effluent from the first and second zones can each be returned or combined and returned to the natural source from where the liquid was obtained or can be recycled for use as fresh water. The outlet stream from the second zone can be directed to the first zone to insure that this portion of the water receives the full ozone treatment at all times. The gaseous stream removed from the upper portion of the second ozonation zone is then utilized for the regeneration of the ozone-containing gas by mixing with oxygen, drying, bleeding to remove nitrogen and then introducing it to an ozone-generation device.
The present invention, on the other hand, deals with an efficient method of treating a cyanide-containing aqueous liquor with an ozone-containing gas in at least one ozone-contacting zone to destroy cyanides contained therein by providing a high-mass transfer of ozone to the aqueous liquor.
The high mass transfer of ozone to influent is accomplished by employing a turbine gas injector in each zone. The injector spins a portion of the wastewater influent in a turbine-bladed impeller rotor at high speed and mixes the wastewater with an ozone-containing gas, which gas is broken down into small bubbles by the mixing and distributed in the influent portion. This stream of ozone-containing gas and influent is then injected into the zone containing a volume of wastewater influent.
By recycling at least a portion of treated liquor from the contacting zone to either the influent liquor stream or to the zone itself, or by retaining said portion in a holding zone and then recycling, additional cyanide as well as cyanate, formed as an intermediate in the cyanide oxidation, is also oxidized by the direct ozone treatment in the zone and the residual oxidants in the recycled liquor.
When carried out in two or more ozone-contacting zones wherein at least a portion of an ozone-containing gas from an ozone source contacts liquor in the latter zones and the expended gas therefrom is used as at least a portion of the ozone treating gas for the earlier stages, with any remainder of treating gas being supplied by the ozone source, substantially complete ozone utilization is achieved in destroying cyanide.
By recycling or retaining and recycling at least a portion of treated effluent from one or more latter zones to an earlier zone or to incoming untreated liquor itself, increased cyanide and cyanate oxidation is accomplished. In addition, certain free and complexed metals in the wastewater such as copper, iron and zinc are oxidized to an insoluble and filterable or settleable state.
A process for substantially destroying cyanate which is formed by the oxidation of cyanide by any of the cyanide-treatment processes of this invention is also provided.